Fixes #3 CLS event values sent to GA aren't useful.

[laradevitt]

I thought it might be useful to see the original event value when debugging, so I've added it to the onDebug() output.

I have also updated the README, since the CLS values will need to be converted in GA reports.

To minimize confusion it might be best to apply this PR to a new major version which also updates the web-vitals version dependency (which may of course require further changes to be compatible).